Process for the production of metallic borides on the surface of metals



United States Patent 3,029,162 PROCES FOR THE PRODUCTION OF METALLIC BQRIDES ON THE SURFACE OF METALS Robert Lionel Samuel, Acton, London, and Norman Anthony Loclrington, Loughton, England, assignors to Chromalloy Corporation, White Plains, N.Y., a corporation of New York No Drawing. Filed May 21, 1959, Ser. No. 814,692 6 Claims. (Cl. 117-107) This invention relates to a new process for the production of metallic borides on the surface of metals.

There has been, in recent years, considerable development in the production and commercial use of metallic borides. In particular, these compounds have been shown to possess outstanding properties from the point of view of hardness, wear resistance, thermal oxidation and corrosion resistance.

While boron additions to steel and commercial alloys usually result in some improvement of physical properties, it is not possible to exceed very low concentrations of boron, without altering deeply the structure and in creasing the brittleness of the material.

As a consequence, the possibility of creating surface coatings containing high concentrations of boron or borides is of considerable practical interest. It is possible to diffuse boron into iron, steel, nickel and other metals by reaction at elevated temperature of a boron halide upon the surface of the metal to be treated. However, the physical and chemical properties of the borides of iron and nickel are far less valuable than those of elements such as molybdenum, tungsten, chromium, silicon and titanium.

According to the invention, there is provided a process for the production of a high concentration of metallic borides on the surface of another metal, wherein a metal is first diffused with a second metal and subsequently subjected to boron diffusion; thus, for example, steel, nickel, copper or molybdenum may be diffused with, for example, chromium, molybdenum, tungsten, silicon or titanium, and then subjected to boron diffusion.

A steel surface can thus be provided With a coating rich in chromium boride by subjecting the steel first to a chromizing process which creates a diffusion coating with a high chromium content and, then, to a boron diffusion which converts part or all of this chromium into a boride compound.

A simplified flow diagram illustrative of a technique embodying and for practicing this invention is as follows:

Metal article coated with a 1st metallic coating material Coated article in pack contaming source of boron and halide Heating coated article in boron pack for diffusion coating containing boride of 1st coating material The following examples are given for the purpose of illustrating the invention.

Example 1 A piece of low carbon steel, 1" length x /2" diameter was chromized by packing in a mixture containing 40% chromium powder, 60% alumina powder and 1 part per 1000 of said mixture of ammonium chloride, and heating 3,029,102 Patented Apr. 10, 1962 ice in a sealed vessel to a temperature of 1050 C. for 4 hours.

This treatment resulted in a coating being formed on the surface of the steel, consisting of a diffused layer ap proximately 0.003" deep having an average chromium content of 22% by weight.

The hardness of the treated steel was 150 VPN and that of the chromized layer, 220 VPN.

The sample was then packed in a tube, in contact with a composition containing 5% boron powder, 7 /z% chromium powder, 87 /2% alumina powder and 1 part per 1000 of said composition ammonium fluoride. The tube was heated at a temperature of 1000 C. for 4 hours. The treated specimen had the following characteristics: the zone of diffusion had extended to approximately 0.0035. There was a sharply defined needle-shaped phase extending throughout the coating but more concentrated near the surface. The hardness of the coating near the steel interface was of the order of 450 VPN While it reached figures in excess of 1400 VPN at or near the surface.

Example 2 4 hours.

This treatment resulted in a diffusion coating at the surface of the molybdenum cylinder of approximately 0.0025" depth. The average chromium content of the coating was and the hardness of the coating was of the order of 350 VPN.

The specimen was then processed with a boron composition as in Example 1, but the 1 part per 1000 of said composition of ammonium fluoride was replaced by an equivalent weight of iodine.

As a result of this second treatment, the depth of coating had not altered but it now consisted entirely of chromium boride near the surface, for approximately half of the total depth of the original chromium-diffused layer. The hardness at the surface was of the order of 1500 VPN.

Example 3 A low carbon steel specimen 1" x /z" diameter was siliconized by packing in a mixture containing 5% silicon powder, 6% chromium powder and 89% alumina powder to which 2 parts per 1000 of said. mixture of ammonium fluoride had been added. The sealed container was heated at 1050 C. for 4 hours. i

As a result of the treatment, the steel specimen had a ditfused coating of approximately 0.005 thickness with a silicon content of the order of 15% and a small concentration of chromium (under 0.5%). The hardness of the coating was VPN.

The sample was then processed with a boron composition identical with that used in Examples 1 and 2, but the ammonium fluoride or iodine was replaced by 2 parts per 1000 of ammonium chloride.

Examination of the specimen after final processing showed that the total depth of diffusion had increased to 0.006" and that there was considerable concentration of bon'de phases at and near the surface. The hardness was in excess of 1500 VPN.

Example 4 A piece of low carbon steel similar to that of Example 3 was packed in a mixture of 20% titanium sponge and 80% titanium oxide to which 1 part per 1000 of said mixture of iodine had been added. The whole was packed in a steel tube and heated to a temperature of 1100 C. for 4 hours, while maintaining a partial vacuum in the tube. After removal from the tube, the specimen had a difliused coating of titanium of approximately 0.002" depth. The hardness of the coating was 250 VPN.

The specimen was then boron diffusedas in Example 3 but, in this case, the halide was provided by 1 part per 1000 of the mixture of ammonium bromide. After treatment, the coating thickness was unchanged but there was a high concentration of iron and titanium borides at and near the surface. The hardness was in excess of 1500 VPN.

The invention is not limited to the examples given above. Thus, metals to be treated include: iron, nickel, cobalt, molybdenum, tungsten, titanium, Zirconium, copper and alloys thereof.

The first coating may be obtained by diffusing one or more of the following elements: chromium, silicon, aluminum, vanadium, titanium, zirconium, molybdenum, tungsten, columbium and tantalum.

We claim:

1. In a process for the diffusion coating of boron into a surface of metal articles of the character described, which comprises the steps of diffusion coating into said metal articles a metallic material other than boron, thereafter heating said thus coated metal article in a coating pack including a source of boron and a volatilizable halide as a carrier for said boron for effecting diffusion coating of said boron into the surface of said coated metal article.

2. In a process for the diffusion coating of boron into a surface of metal articles of the character described, which comprises the steps of diffusion coating into said metal articles a metallic material other than boron and selected from the group consisting of chromium, silicon, aluminum, vanadium, titanium, zirconium, molybdenum, tungsten, columbium, and tantalum, thereafter heating said thus coated metal article in a coating pack including a soruce of boron and a volatilizable halide as a carrier for said boron for effecting diffusion coating of said boron into the surface of said coated metal article.

3. In a process for the diffusion coating of boron into a surface of metal articles of the character described, which comprises the steps of diffusion coating into said metal article a metallic material other than boron, said coating being accomplished by a direct pack impregnation diffusion coating step including the embedding of said article to be coated in a pack of a source of said metallic material and a volatilizable halide carrier therefor, thereafter heating said thus coated metal article in another coating pack including a source of boron and a volatilizable halide as a carrier for said boron for effecting diffusion coating of said boron into the surface of said coated metal article.

4. In a process for the diffusion coating of boron into a surface of metal articles of the character described, which comprises the steps of diffusion coating into said metal articles a metallic material other than boron, thereafter heating said thus coated metal article in a coating pack including a source of boron and a source of said metallic material of said first coating and a volatilizable halide as a carrier for said boron for effecting diffusion coating of said boron into the surface of said coated metal article.

5. In a process for the diffusion coating of boron into a surface diffusion coating on metal articles of the character described to provide therein a boride of a constituent of such coating different from a boride of the metal of which said metal article is composed, the steps which comprise subjecting said metal article with a coating of a metal other than the metal of which said metal article is composed to a diffusion coating step for diffusing boron into said coating for forming therein a boride of said metal of said coating by embedding said coated metal article in a diffusion coating pack including a source of volatile halide and a source of boron, and heating said article as embedded in said pack for the diffusion coating of said boron into said preexisting metallic coating on said metal article to form said boride coating thereon.

6. In a process for the diffusion coating of boron into a surface diffusion coating on metal articles of the character described to provide therein a boride of a constituent of such coating different from a boride of the metal of which said metal article is composed, the steps which comprise subjecting said metal article with a coating of chromium as a metal other than the metal of which said metal article is composed to a diffusion coating step for diffusing boron into said coating for forming therein a boride of said chromium coating by embedding said chromium coated metal article in a diffusion coating pack including a source of volatilizable halide and a source of boron, and heating said article as embedded in said pack for volatilizing said halide and diffusion coating of said boron into said pro-existing metallic coating on said metal article to form said boride coating thereon.

References Cited in the file of this patent UNITED STATES PATENTS 1,987,576 Moers Ian. 8, 1935 2,683,305 Goetzel July 13, 1954 2,690,409 Wainer Sept. 28, 1954 2,852,409 Roe Sept. 16, 1958 2,930,106 Wrotnowski Mar. 29, 1960 FOREIGN PATENTS 19,461 Great Britain 1912 1,024,306 Germany Feb. 13, 1958 OTHER REFERENCES N.S. Gorbunov: Diffuse Coatings on Iron and'Steel, The Academy of Sciences of the U.S.S.R., Moscow (1958), pages to 89 of translation relied on.

Powell et al.: Vapor-Plating, John Wiley 8: Sons, New York (1955), pages 106-108 relied on. 

2. IN A PROCESS FOR THE DIFFUSION COATING OF BORON INTO A SURFACE OF METAL ARTICLES OF THE CHARACTER DESCRIBED, WHICH COMPRISES THE STEPS OF DIFFUSION COATING INTO SAID METAL ARTICLES A METALLIC MATERIAL OTHER THAN BORON AND SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, SILICON, ALUMINUM, VANADIUM, TITANIUM, ZIRCONIUM, MOLYBDENUM, TUNGSTEN, COLUMBIUM, AND TANTALUM, THEREAFTER HEATING SAID THUS COATED METAL ARTICLE IN A COATING PACK INCLUDING A SOURCE OF BORON AND A VOLATILIZABLE HALIDE AS A CARRIER FOR SAID BORON FOR EFFECTING DIFFUSION COATING OF SAID BORON INTO THE SURFACE OF SAID COATED METAL ARTICLE. 